From the discovery of the X-ray in 1895 until now, the emission of a radiation ray, at any energy range, is essentially divergent and the intensity is a function of the distance between it and the emission source (inverse square law). This is due to the X-ray production mechanism; in other words, electrons that impact a target. There are currently several ways to generate X-ray beams, each with a determined source size and a specific, always-positive divergence. The X-ray beams employed in radiotherapy are divergent.
The expected objective of radiotherapy is, by using X-rays, to achieve a high X-ray flow zone within a specific volume. These X-rays would then deposit their energy. The energy deposited per unit mass is known as dose in radiotherapy. Since the beam that is used is noticeably divergent, several beams (fields) aimed at the volume of interest must be employed. As is widely known, the depth dose for an X-ray beam is dependent on an exponential downward curve according to the depth, with a maximum value near the surface. A multi-field application allows for a maximum dose in the interest volume (tumor site), despite the fact that the dose values in the surrounding areas are lower than those at the tumor site. These dose values are significant as they have higher values than what is tolerable in some cases, which can prevent the use of an effective dose in the tumor.
More refined techniques such as Intensity Modulated Radio Therapy (IMRT) or arc therapy improve and conform the maximum flow volume of X-rays, thus lowering dose levels in neighboring tissues and organs, though this decrease is not significant. A dose value decrease of up to 80% in tissues and organs near the interest zone has currently been achieved in relation to the dose in the interest zone. Treatment planning, however, continues to be complex. A decrease of collateral effects caused by radiation is always attempted, though their complete elimination is impossible.
A radiotherapy technique that has lower collateral effects and greater radiobiological effectiveness at the tumor site is that of hadrontherapy. This technique uses hadrons (protons or heavier nuclei) to deposit high doses at the tumor site that are very conformed, that is to say, limited to that area. The cost of this technique, however, is much higher than conventional photon or electron methods, precluding its use for many patients. It is also rarely available at hospitals and health and treatment centers. FIG. 1 shows a sketch comparing the relative depth dose of the most widely used radiotherapy techniques.
This invention proposes the use of a device able to generate a convergent photon beam with advantages that are significantly greater than the conventional external radiotherapy technique and the hadrontherapy techniques, the latter catalogued as being those that provide better results.
From a comparative point of view, conventional conformal radiotherapy techniques or IMRT (the latter being better): administer a greater superficial dose; are a greater risk to healthy organs; require fractioning and a more complex planning system; require more energy and, therefore, more costly bunkers; not all tumors are accessible; thus rendering the techniques less effective. The advantages of these techniques are that a greater volume is treated and the positioning system is simpler. FIG. 2 shows the fundamental difference between the conventional method, (a), and the convergent method (b).
The convergent method, however, presents: lower surface dose, low dose in healthy organs; high dose in the tumor which does not require fractioning; simpler planning system; shorter treatment (one or two sessions); greater effectiveness and accessibility to most tumors; simpler refrigeration system; high energy is not required thus bunker shielding requirements are lower. The disadvantage is that, as the treated volume is smaller, a tumor scan and a more precise positioning system are required.
The only external photon method that is comparable, quality-wise, to the convergent technique of the invention being proposed is the arc therapy technique, also known as Tomotherapy, using photons with a linear accelerator (LINAC) that generates electron beams to produce the required X-rays. Arc therapy emulates convergence by using an angular scan around the isocenter (tumor site). Despite longer sessions and equally complex planning, however, each beam is still essentially divergent and the doses in healthy organs are not insignificant. Like the other conventional LINAC techniques, several sessions are required. Similar results can be obtained using a robotic device called a “Cyberknife”.
The hadrontherapy technique presents the following: a low surface dose and is highly effective as it deposits a high dose depth in a very small site (Bragg peak, see FIG. 1). Hadrons and ions have high radiobiological effectiveness (protons are 5 times more effective than photons) and complex positioning systems. However, a very complex installation is required, which includes a synchrotron able to accelerate particles to energies ranging from several hundred MeV to several GeV, high vacuum, and electrical and magnetic guide systems. Furthermore, the cost of a hadrontherapy system exceeds $100M USD. There are 28 hadrontherapy installations in the world's most developed nations and the technique continues to grow despite its high cost. Hadrontherapy is out of the question for Chile at present though Spain is evaluating the possibility of acquiring one of these installations in the next few years. Hadrontherapy has shown excellent results in patients with complex cancers as it is able to treat tumors that cannot be treated with photons. The cost of this therapy, however, means it is available to only a select few.
The convergent method employed by the invention presented here delivers low surface dose and is highly effective, as it deposits a high depth dose in a very small area (“peak focus”). Photons have less radiobiological effectiveness, but the dose deposited at the focus peak site can be up to 100 times greater than the dose on the surface, despite the attenuation effect. This compensates for the photons' lower radiobiological effectiveness and generates an even lower relative dose on the surface and in the healthy organs than that which is obtained in hadrontherapy. The positioning system, however, must be more precise than that of conventional techniques. All of the above will allow for the treatment of complex cancer cases as with hadrontherapy but with a less complex installation.
Furthermore, the cost of a LINAC plus a bunker and control building is in the $2 to 3 MUSD ranges, while a LINAC-adaptable convergence system may cost $0.5M USD or less, a noteworthy advantage in relation to the cost of a hadrontherapy installation which is almost two orders of magnitude greater. In this regard, a convergent system would function similarly to a hadrontherapy system but at a significantly lower cost.
The first step taken prior to the development of this invention was the study of the effects of a photon beam's convergence on a specific material that was carried out by Monte Carlo Simulations (MCS) and theoretical calculations. FIG. 3 shows the curves of a depth dose corresponding to MCS and the theoretical results.
Devices currently exist that achieve beam convergence with a divergent X-ray beam based on the total reflection principle. The divergent X-rays enter a cone-shaped capillary, and the beams travel the length of it by total reflection inside the capillary until they reach the end. The exit section is smaller than the entry section, thus allowing a greater intensity to be achieved. In order to attain a significant increase in intensity, a set of these cone-shaped capillaries set in parallels is used. This makes up what is known as a poli-capillary and allows the entry area to be increased. However, as these devices employ the total reflection principle, its use is only advantageous with X-rays with energies below 50 keV, which limits its application in radiotherapy equipment, where the X-ray energy is much greater than the aforementioned amount. There is currently a great variety of X-ray focusing devices that use not only the total reflection principle but diffraction and/or refraction as well, though all of them can be used for low energy X-rays (<50 keV). For example, in astronomy, an X-ray telescope (Chandra and equivalents) obtains X-ray images of the Universe, allowing us to see emission sources, including black holes. This is a large-scale device (several meters) that is based on the same total reflection principle and uses reflector plates and other materials.